1. Field
Embodiments of the present invention generally relate to the field of current feedback amplifiers.
2. Background
Current feedback amplifiers (CFAs) offer excellent performance, especially in terms of slew-rate and bandwidth, and are widely used in a number of high speed applications. However, one drawback of conventional CFA topology is the poor input common mode voltage range (CMVR). For example, due to architectural limitations, conventional CFAs are not able to operate properly at input voltages that are at or near the supply rails. In other words, conventional CFAs do not have “rail to rail” input stages, which limits application of CFAs to relatively high supply voltage (≧5V).
The CMVRs of conventional input topologies are limited to at least two base emitter voltages (Vbe, approximately 700 mV each) plus two saturation voltages (Vsat, approximately 200 mV each). For example, FIG. 1 illustrates one conventional input stage topology 100 that employs a complementary cascaded emitter follower stage. As illustrated in FIG. 1, the CMVR with respect to the input +In is limited by the base-emitter voltage of Qcp1 and the saturation collector-emitter voltage of Qbp1 with respect to the positive supply rail VCC and by the base-emitter voltage of Qcn1 and the saturation collector-emitter voltage of Qbn1 with respect to the negative supply rail VEE. In other words the CMVR of input stage 100 is:CMVR=VCC−VEE−2VBE−2VSAT  (1)Thus, in a 5V application, the CMVR of input stage 100 is approximately 3.2V. However, it is appreciated that, depending on component selection, the CMVR may be as much as 3.4V or limited to as little as 2.5V, a loss of roughly half the supply range. It should be appreciated that a similar CMVR may be derived for the −In input of input stage 100.
FIG. 2 illustrates another conventional current feedback amplifier input stage 200 that also employs a complementary emitter follower stage. Similar to FIG. 1, FIG. 2 illustrates that the CMVR of the input +In is reduced by the base-emitter voltage of Q59 and the saturation collector-emitter voltage of Q52 with respect to the positive supply rail VCC and by the base-emitter voltage of Q60 and the saturation collector-emitter voltage of Q55 with respect to the negative supply rail VEE. Thus, the equation for the CMVR of input stage 200 is also expressed in Equation 1 above. Therefore, input stage 200 also experiences the same flaws of prohibitive CMVR limitations at low supply voltages. In FIGS. 1 and 2, the signals vbp and vbn are bias voltages that set up appropriate currents in the transistors connected to these voltages.
Moreover, it should be appreciated that in practice, input stages 100 and 200 also include degeneration resistors (not shown) between the bias current and mirror transistors and the supply rail, which further cut into the CMVR of the input stages 100 and 200. Furthermore, input stages 100 and 200 may be implemented with a MOS architecture, in which case the inputs would require a voltage headroom consisting of a Vgs and a Vds,sat, roughly 800 mV from each rail.
More recently, input stages have been introduced for current feedback amplifiers that employ a closed-loop buffer as part of the input stage. However, these input stages similarly do not allow for rail to rail CMVR at the inputs of a current feedback amplifier.
Thus, conventional current feedback amplifier technology lacks a true rail to rail input stage.